Continuous Integration

Hello!

I was watching the repo for few months and noticed a relatively big number of issues with basic setup / run. Recent examples are #156 #158

One technique that can help reduce the number of such problems in the future is to employ Continues Integration. It will allow to run setup and simple run steps on all interesting platforms (macOS, linux, Windows) on all commits, make sure that basic scenarios work and enable adding unit tests in the future.

There are popular free options for OSS projects: Travis CI, AppVeyor. Travis is currently one of the most popular options for macOS builds, but it doesn't support Windows, so most projects doing builds on both these CIs. I would be happy to help with setting up it for manim, but it would require a little bit of collaboration from the maintainers, because only admins of the repo can create the CI integration webhooks.

In both AppVeyor and Travis, CI steps can be defined in the form of `yaml` file in the root of the repo.
For examples, see
https://github.com/PowerShell/PowerShell/blob/master/.travis.yml
https://github.com/PowerShell/PowerShell/blob/master/appveyor.yml

So the path to enabling CI for manim could be the following:
1) I will create the `appveyor.yml` and `.travis.yml` in my fork and make sure CI works as expected
2) I will send a PR to this repo to add these 2 files
3) One of the maintainers would create builds integration and we will test the builds on the PR from (2)
4) PROFIT

If this is something that you are interested it, let me know and I will work on (1).